This invention relates to analog-to-digital converters, and more specifically relates to such converters which employ integrating circuits.
A variety of integrating circuit-type analog-to-digital converters have been devised in the past. In such a converter, the integrating circuit generally comprises an operational amplifier having a capacitive feedback network. At the beginning of a conversion cycle, the capacitor is discharged and the inverting input of the operational amplifier is connected to the input analog voltage being converted. The integrator causes a linear voltage increase across the capacitor which continues for a predetermined period of time determined by a digital counter. At the end of the predetermined time period, the counter is reset, the inverting input of the amplifier is connected to a precision reference voltage and the capacitor charges in the opposite direction until a comparator detects that ground potential is reached. At this time, an output pulse is generated which stops the counter. The resulting count in the counter is proportional to the analog input voltage.
The difficulty with this form of converter is that the precision reference voltage must have a polarity opposite the polarity of the input analog voltage. This is of particular importance when the input signal is derived as a ratio of the reference voltage. In this case, both polarities of the precision reference voltage must be available, thereby increasing the parts required for the power supply and adding additional error-causing circuitry. In many instances, the only circuitry in a system requiring the use of precision reference voltages of opposite polarities is the analog-to-digital converter. As a result, expensive and potentially unreliable circuitry must be added to the power supply in order to accommodate this single circuit component.
In order to overcome the disadvantages of the prior art, the applicants have discovered an integrating-type analog-to-digital converter which requires only a single polarity for the input analog voltage and the reference voltage. In order to achieve this result, according to a preferred feature of the invention, the applicants have provided an input terminal for receiving the analog DC voltage and a current sink, such as a source of ground potential, having a neutral polarity and a neutral voltage. The applicants also provide a source of a reference voltage which may have a polarity identical to the polarity of the analog voltage. An integrating circuit is provided having an inverting input, a non-inverting input, and an output for producing an output signal. Comparator means produce an indicating signal in response to a predetermined relationship between the output signal and the reference voltage. A digital counter resettable in response to the indicating signal is also provided, and a pulse generator generates clock pulses which may be transmitted to the counter. First and second switch means control the connections between the analog voltage, current sink, and reference voltage. The operation of the switch means and counter is coordinated by a control circuit so that the counter produces a digital number proportional to the magnitude of the analog voltage by using a reference voltage of the same polarity.
According to another feature of the invention, the inverting input of the integrating circuit is periodically disconnected from the analog voltage, reference voltage and current sink so that the output signal can slew to a new value of reversed polarity without reversing the polarity of an input signal.